The present invention relates to a method of printing a substrate with an inkjet printing device comprising at least one print head provided with at least one row of nozzles, wherein substantially fixed locations on the substrate, which locations form a regular field of pixel rows and pixel columns, are provided with ink drops image-wise, the resolution of the pixel columns being equal to the resolution of the row of nozzles. The method comprises a first printing stage in which a strip of pixel rows is provided with ink drops, whereafter the print head is displaced in a direction substantially parallel to the pixel columns, and a second printing stage in which the strip is provided with supplementary ink drops. The present invention also relates to a printing device suitable for the use of this method.
A method of this kind is known from U.S. Pat. No. 5,640,183. A known problem in inkjet printing devices is that deviations of individual nozzles result in ink drops leaving such nozzles at the wrong angle, so that the ink drops occupy a different place on the substrate with respect to the center (the normal position) of the fixed locations (“pixels”). As a result, disturbing faults can occur in a printed image. This method is based on a redundancy strategy in order to mask such printing faults. In this method, a strip of pixel rows of the substrate is printed in two stages with a print head with which the resolution of the row of nozzles, i.e. the number of nozzles per unit of length, is equal to the resolution of the pixel columns, i.e. the number of locations per unit of length in a direction parallel to the columns. In each stage, a number of locations of the pixel rows of the strip are printed with ink drops such that all the ink drops together form the image for printing within the strip. The known strategy now is such that the row of nozzles comprises a number of extra nozzles, typically six out of a total of 106 nozzles. In the first stage, a first set of ink drops is printed with a sub-row of a size of 100 adjoining nozzles, selected from the complete row. In the second stage, a second set of ink drops is printed with a second sub-row, again consisting of 100 adjoining nozzles. The first and second sets of ink drops together form the image for printing (within said strip).
By now selecting the second sub-row at random from the entire row (in this case there are therefore seven options, i.e. the sub-rows starting with the nozzles 1, 2, 3, 4, 5, 6 or 7 and ending with the respective nozzles 100, 101, 102, 103, 104, 105, 106, 107), any printing faults as a result of deviations in the ejection of ink drops are distributed at random as far as possible over the different strips of the substrate, so that they are barely visible, if at all, to the human eye.
A disadvantage of such a method is that a number of nozzles is not used in each stage, so that the maximum productivity of the printing device is smaller than would be possible based on the total number of nozzles. A following more important disadvantage is that the print head must be very accurately displaced, prior to the second printing stage, with respect to the substrate over a distance which, depending on the choice of the second sub-row of adjoining nozzles, varies with the width of 0.1 or a number of pixel rows (rising to 6 in the example described). A shift of this kind is achieved by displacing the paper by means of a motor. These small but very accurate shifts which are selected at random mean that the accuracy of the paper transport must meet stringent requirements.